In recent years a new high-speed communication service referred to as LTE (Long Term Evolution) has been expected as a standard for communication by a mobile station such as a portable telephone. In addition, a LTE-advanced system which is a further developed version of LTE is discussed in 3GPP (3rd Generation Partnership Project).
Furthermore, the LTE-advanced system is to be proposed as an IMT-advanced system which is a further developed version of an IMT (International Mobile Telecommunication)-2000 system which ITU-R (International Telecommunication Union Radio communications sector) determines to discuss.
W-CDMA (Wideband-Code Division Multiple Access), CDMA one, and WiMax (Worldwide Interoperability for Microwave Access) are typical IMT-2000 systems.
With a LTE-advanced system introducing a MBSFN (Multimedia Broadcast multicast service Single Frequency Network) in which MBMS (Multimedia Broadcast Multicast Service) data is transmitted and a relay apparatus (relay node) for performing radio relay with a LTE system as a base is discussed (expansion of uplink/downlink bandwidth, introduction of uplink MIMO (Multiple Input Multiple Output), and the like are also discussed). Description will now be given with a LTE-advanced system as an example.
(1) MBMS and MBSFN
A MBMS is a service for broadcasting data to unspecified or specific users. To be concrete, broadcasting information such as news or multicasting information to specific users is possible.
Furthermore, A MBSFN in which a plurality of base stations transmit MBMS data in synchronization with one another by the use of the same resource is discussed as a method for transmitting broadcast data (MBMS data) by the use of a MBMS.
“SFN” (Single Frequency Network) of “MBSFN” means using the same radio frequency. That is to say, usually a transmission area (MBSFN are) is set in a MBSFN and the same radio frequency is used in that area (see TS36.300V8.6.0 15 MBMS).
Moreover, with a MBSFN a plurality of base stations transmit the same data at the same frequency at the same timing. As a result, a mobile station can receive MBMS data transmitted from the plurality of base stations.
The reason for this is as follows. If delay time is shorter than or equal to the length of a CP (Cyclic Prefix) in, for example, OFDM (Orthogonal Frequency Division Multiplexing) receiving, then plural pieces of data can be received and synthesized. By receiving and synthesizing plural pieces of data, the effect of the improvement of a receiving characteristic can be obtained.
A CP is a redundant portion added at data transmission time to prevent a data overlap, and corresponds to a GI (Guard Interval) in terrestrial digital broadcasting. The length of a CP used in a MBSFN is longer than that of a CP added to unicast data in normal communication.
FIG. 20 illustrates the format of radio data. Radio data includes a CP and data. A CP used at unicast transmission time is referred to as a normal CP and a CP used in a MBSFN is referred to as an extended CP. The length of a normal CP is 4.69 μsec and the length of a CP used in a MBSFN (length of a CP included in MBMS data) is 16.67 μsec.
FIG. 21 illustrates data receiving and combining. It is assumed that a mobile station 120 receives MBMS data (data b) transmitted from a base station B and that the mobile station 120 receives MBMS data (data a) transmitted from a base station A time t after receiving the data b (data a and b are broadcast data and are equal in service contents).
If the delay time t falls within the range of the length of a CP from the time when the mobile station 120 begins to receive the data b, then the mobile station 120 can receive not only the data b but also the data a and combine the data a and b. As described above, a CP is long in a MBSFN. Therefore, a mobile station can also receive MBMS data transmitted from a remote base station (corresponding to the base station A in this example) and can perform combining.
(2) Relay Apparatus (Relay Node)
With a LTE-advanced system a relay node is installed between a base station and a mobile station, for example, for cell extension or as countermeasures for dead spots.
FIG. 22 illustrates cell extension. A mobile station 120 is outside a cell 100a of a base station 100. A relay node 110 is installed within the cell 100a. The mobile station 120 is within a relay area 110a in which the relay node 110 can perform relay.
If a relay node such as the relay node 110 does not exist, the mobile station 120 is outside the cell 100a and cannot communicate with the base station 100. However, if the relay node 110 is installed, the mobile station 120 is within the relay area 110a of the relay node 110. Even if the mobile station 120 is outside the cell 100a, radio relay is performed via the relay node 110 and communication can be performed between the base station 100 and the mobile station 120.
FIG. 23 illustrates countermeasures for a dead spot. A relay node 110 is installed within a cell 100a of a base station 100. There is a dead spot 110b within the cell 100a. A mobile station 120 is in the dead spot 110b. It is assumed that a relay area 110a of the relay node 110 covers the dead spot 110b. 
If a relay node such as the relay node 110 does not exist and the mobile station 120 is in the dead spot 110b, it is difficult for the mobile station 120 to communicate with the base station 100. However, if the relay node 110 is installed and the relay area 110a of the relay node 110 covers the dead spot 110b, then radio relay is performed via the relay node 110 and communication can be performed between the base station 100 and the mobile station 120 in the dead spot 110b. 
The following technique is proposed in Japanese Laid-open Patent Publication No. 2008-503130 (Paragraphs [0015]-[0020], FIG. 1) as a conventional technique regarding a MBMS. A mobile station estimates cell quality on the basis of the difference in transmission power between a common pilot channel and a common control channel and receives data from an adjacent cell in which cell quality is the highest.
In addition, the following technique is proposed in Japanese Laid-open Patent Publication No. 10-032557 (Paragraphs [0019]-[0021], FIG. 1) as a conventional radio relay technique. A transmission apparatus hierarchizes and transmits a relay apparatus signal which a relay apparatus retransmits and a receiving apparatus signal transmitted directly to a receiving apparatus. The relay apparatus demodulates the relay apparatus signal, modulates it again, and retransmits it.
With a MBMS radio network, as described above, a relay node can be installed for performing cell extension or taking countermeasures for a dead spot. In addition, with a MBSFN a radio signal is transmitted by the use of an extended CP which is longer than a normal CP used for normal unicast transmission. Accordingly, a radio signal transmitted from a base station distant from a mobile station can be received via a relay node. As a result, the possibility of receiving and combining more pieces of data can be enhanced.
With a conventional MBMS radio network, however, the problem of being unable to distinguish between unicast data and MBMS data transmitted in a MBSFN exists.
FIG. 24 illustrates the problem of being unable to distinguish between unicast data and MBMS data. There are base stations 101 through 103, mobile stations 121 through 123, and a relay node 110. The base station 101 transmits unicast data r1 to the mobile station 121. The base station 103 transmits unicast data r3 to the mobile station 123. In addition, the base station 102 transmits MBMS data r2 to the relay node 110 and the relay node 110 relay-transmits the MBMS data r2 to the mobile station 122.
With unicast data transmission the base station scrambles unicast data so that the unicast data can be distinguished from another piece of unicast data transmitted by the use of the same radio resource. That is to say, by using scrambling codes which differ in initial value, the unicast data can be distinguished from another piece of unicast data transmitted by the use of the same radio resource. Accordingly, the unicast data r1 and r3 indicated in FIG. 24 can be distinguished. In addition, with MBSFN transmission plural pieces of MBMS data are transmitted so that they can be distinguished. Therefore, pieces of MBMS data can be distinguished. That is to say, if the same communication format is used, pieces of data can be distinguished.
However, unicast data and MBMS data differ in communication format. In addition, there is no express provision that unicast data and MBMS data differ in scrambling code initial value. Accordingly, there is no guarantee that unicast data and MBMS data can be distinguished by scrambling codes. Furthermore, unicast data and MBMS data may be transmitted at the same time by the use of the same radio resource. As a result, in an environment in which unicast data and MBMS data mingle, it may be impossible to distinguish between them.
To be concrete, there is no guarantee that a scrambling code for a PDSCH (Physical Downlink Shared Channel), which is a radio channel used for transmitting user data in unicast communication, and a scrambling code for a PMCH (Physical Multicast Channel), which is a radio channel used for transmitting user data in MBSFN transmission can be distinguished. As a result, it may be impossible to distinguish between a PDSCH and a PMCH. This may cause interference.
In the case of FIG. 24, it is assumed that the mobile station 121 is at a position where the mobile station 121 can receive both the unicast data r1 and the MBMS data r2 and that the mobile station 123 is at a position where the mobile station 123 can receive both the unicast data r3 and the MBMS data r2.
In this environment, the mobile station 121 or 123 which originally wants to receive unicast data is unable to distinguish MBMS data r2 transmitted from the relay node 110, so that the MBMS data r2 becomes an interference wave.
On the other hand, even if unicast data and MBMS data can be distinguished for a certain period of time, base stations or a base station and a relay node are not necessarily synchronized. Accordingly, timing at which scrambling begins, for example, in one base station gradually deviates from timing at which scrambling begins in the other base station. This degrades code identification capability. As a result, it is impossible to distinguish a PDSCH and a PMCH, and interference occurs.
FIG. 25 illustrates the occurrence of interference caused by a timing deviation. A black slot indicates MBMS data in MBSFN transmission and a white slot indicates unicast data. In a state in which transmission sequences A1 and B1 can be distinguished, two pieces of MBMS data are in the same timing, for example, at a timing T1. Accordingly, the two pieces of MBMS data can be distinguished and interference does not occur. Two pieces of unicast data are in the same timing at a timing T2. Accordingly, the two pieces of unicast data can be distinguished and interference does not occur.
On the other hand, it is assumed that the transmission sequence A1 changes to a transmission sequence Ala due to a timing deviation. In this case, MBMS data and unicast data are in the same timing in the transmission sequences Ala and B1 at each of timings T3 through T6. Accordingly, the MBMS data and the unicast data cannot be distinguished and interference occurs. This degrades the transmission characteristics of one or both of the MBMS data and the unicast data, resulting in degradation in transmission quality.